1. Field of the Invention
The present invention generally relates to polyolefin fibers such as fibers comprising polypropylene. In particular, the present invention relates to processes and apparatus for the production of polyolefin fiber. The present invention also relates to fibers produced using compact long spin processes and apparatus, and to articles incorporating these fibers.
2. Background
The production of polymer fiber and filaments usually involves the use of a single polymer or blend of polymers optionally mixed with nominal amounts of stabilizers, pigments and/or other additives such as Elvax.RTM. or Kemamine.RTM.. The mix is extruded into fibers and fibrous products using conventional commercial processes. Nonwoven fabrics are typically made by making a web comprising continuous or staple fibers and bonding the fibers together. For instance, staple fibers may be converted into nonwoven fabrics using, for example, a carding machine, and the carded web is then bonded, e.g., thermally bonded, by any of various techniques, including those utilizing heated rollers, lasers or ultrasonic welding.
Production of fibers through melt extrusion is generally done in either a two-step "long spin" process, or a one-step "short spin" process. The long spin process for polypropylene involves first melt-extruding fibers at typical spinning speeds of about 500 to 3,000 m/min, and more usually from about 500 to 1,500 m/min. The hot extrudate is quenched to form filaments and optionally coated with a finish. These quenched filaments are then collected on, or redirected by, a take-up roll. The spin height, i.e., the vertical distance from the spinnerette to the take-up roll, is typically about 6 m or more in the traditional long spin process. Additionally, in a second step of the draw process, usually run at about 100 to 500 m/min, these fibers may be drawn, crimped, and/or cut into staple fiber.
In contrast, the one-step short spin process involves conversion from bulk polymer to staple fibers in a single step where typical spinning speeds are in the range of about 50 to 200 m/min. The productivity of the short spin process is increased with the use of about 5 to 20 times the number of capillaries in the spinnerette compared to that typically used in the long spin process. For example, while each spinnerette in a typical commercial long spin process might include about 50 to 4,000 capillaries, preferably about 700 to 3,500 capillaries, spinnerettes for a typical commercial short spin process would include about 5,000 to 120,000 capillaries, preferably about 15,000 to 70,000 capillaries. The distance from the spinnerette to the take-up roll in the short spin process is typically about 2 m. When either process is used in the production of bi- or other multi-component filaments, the number of capillaries refers to the number of filaments being extruded, and usually not the number of holes for polymer extrusion in the spinnerette.
The short spin process for the production of fiber is also significantly different from the long spin process in terms of the quenching conditions needed for spin continuity. For example, in a short spin process performed at a spinning speed of about 200 m/min and including a short take-up distance and high hole density spinnerettes, a high quench air velocity, about 900 to 2,500 m/min, is required to complete the fiber quenching within about 2-3 cm of the spinnerette face. In contrast, in the long spin process, with spinning speeds of about 1,000 to 1,500 m/min for polypropylene filament, a much lower quench air velocity in the range of about 60 to 155 m/min is used.
A number of modern uses have been found for nonwoven materials produced from melt spun filaments, such as those produced from a long spin process described above. The filament thus formed may be cut into staple fiber and formed into nonwoven fabrics or used as filler material. Alternatively, the fiber or filament may be used as a continuous fiber or filament in woven or nonwoven fabrics. Other uses of these filaments are known to those of ordinary skill in the art and will not be detailed here. Many of these uses demand special properties of the fiber and corresponding nonwoven fabric, such as special fluid handling, high vapor permeability, softness, strength, integrity and durability. Also, the processing techniques are desired to be efficient and cost effective.
Various techniques are known for producing fibers that are able to be formed into nonwoven materials having superior properties, including high cross-directional strength and softness. For example, U.S. Pat. Nos. 5,281,378, 5,318,735 and 5,431,994 to Kozulla. and European Patent Application No. 719 879 A2 (published Mar. 7, 1996), assigned to Hercules Incorporated, which are incorporated by reference as if set forth in their entireties herein, are directed to processes for preparing polypropylene containing fibers by extruding polypropylene containing material having a broad molecular weight distribution to form a hot extrudate having a surface, with quenching of the hot extrudate in an oxygen-containing atmosphere being controlled so as to effect oxidative chain scission degradation of the surface. In one aspect of the process disclosed in the Kozulla patents, the quenching of the hot extrudate in an oxygen-containing atmosphere can be controlled so as to maintain the temperature of the hot extrudate above about 250.degree. C. for a period of time to obtain oxidative chain scission degradation of the surface.
As disclosed in these patents, by quenching to obtain oxidative chain scission degradation of the surface, such as by delaying cooling or blocking the flow of quench gas, the resulting fiber essentially contains a plurality of zones, defined by different characteristics including differences in melt flow rate, molecular weight, melting point, birefringence, orientation and crystallinity. In particular, as disclosed in these patents, a fiber produced therein can include an inner zone identified by a substantial lack of oxidative polymeric degradation, an outer zone of a high concentration of oxidative chain scission degraded polymeric material, and an intermediate zone identified by an inside-to-outside increase in the amount of oxidative chain scission polymeric degradation. In other words, the quenching of the hot extrudate in an oxygen containing atmosphere can be controlled so as to obtain a fiber having a decreasing weight average molecular weight towards the surface of the fiber, and an increasing melt flow rate towards the surface of the fiber.
Further, U.S. patent applications Ser. Nos. 08/080,849, 08/378,267, 08/378,271 and 08/378,667 to Takeuchi et al., and European Patent Application No. 0 630 996 to Hercules Incorporated, which are incorporated by reference as if set forth in their entireties herein, are directed to obtaining fibers having a skin-core morphology, including obtaining fibers having a skin-core morphology in a short spin process. In these applications, a sufficient environment is provided to the polymeric material in the vicinity of its extrusion from a spinnerette to enable the obtaining of a skin-core structure. For example, because this environment is not achievable in a short spin process solely by using a controlled quench, such as a delayed quench utilizable in the long spin process, the environment for obtaining a skin-core fiber is obtained by using apparatus and procedures which promote at least partial surface degradation of the molten filaments when extruded through the spinnerette. In particular, various elements can be associated with the spinnerette, such as to heat the spinnerette or a plate associated with the spinnerette, so as to provide a sufficient temperature environment, at least at the surface of the extruded polymeric material, to achieve a skin-core fiber structure.
Nakajima et al. (in "Advanced Fiber Spinning Technology," Woodhead Publishing Ltd., 1994, pages 49-53), which is incorporated by reference as if set forth in its entirety herein, examines the effect of spin height on crystal formation in high-speed spinning of nylon fibers.
There is still a need for nonwoven fabrics with improved properties such as strength, softness and vapor permeability, and there is still a need for fibers and nonwovens with improved strength, durability and efficiency of production.
There is also a need for a melt-spun fiber or filament that exhibits high strength upon bonding of webs comprising the fiber or filament. Similarly, there is a need for melt-spun fiber or filament that exhibits high resistance to thermal shrinkage. There is also a need for a bonded nonwoven fabric, for example, a thermal bonded nonwoven fabric, that exhibits high cross-directional strength.
There is also a need for a process for the production of melt-spun fiber or filament. preferably one which permits a high spin speed while preserving spin continuity. Such a process should preferably be amenable to the production of, for example, mono- or bi-component fibers, mono- or multi-constituent fibers, skin-core fibers and the like.
There is also a need for apparatus for the production of melt-spun fiber or filament, preferably, one which can attain high spin speed while maintaining spin continuity. Such an apparatus may be used to produce, for example, mono- or multi-component fiber, including bi-component fiber, mono- or multi-constituent fibers, fibers with or without a skin-core structure, and the like.